#Fibonacci数列
def Fibonacci(n):
    if n < 1:
        print("输入有误")
        return -1
    if(n == 1 or n == 2):
        return 1
    else :
        return Fibonacci(n - 1) + Fibonacci(n - 2)
result = Fibonacci(20)
print("一共有%d对兔子诞生！" % result)
